Vial Smasher’s rules text has a few points that we’re going to search separately.

Whenever you cast your first spell each turn, Vial Smasher the Fierce deals damage equal to that spell's converted mana cost to an opponent chosen at random.

First Spell Each Turn

First let’s focus on the clause “Whenever you cast your first spell each turn”. That is pretty specific, so to get more inclusive results, let’s simplify it and enter “when cast first spell each turn” into the Rules Text box. Also, since we're looking for a Commander, we add “Legendary Creature” in the Type box.

What do we get?

• Erayo, Soratami Ascendant
• Rashmi, Eternities Crafter
• Vial Smasher the Fierce

Double check to make sure your chosen Partner shows up in the results, because if it doesn’t then something probably got spelled or worded incorrectly.

Now let’s take that list of Commanders, search for them on EDHREC, and see what shows up in the Signature Cards and Top Cards sections.

Erayo, Soratami Ascendant

Just because a Commander is on the banlist doesn’t mean you won’t find decks for it on EDHREC. The results are all for mono blue, but looking closer we have a lot of low CMC instants. There are a few counterspell effects as usual for a mono blue deck, as well as a couple of cards that untap some of your lands when played. There’s a card with the Delve keyword which makes itself cheaper to cast without affecting the card’s CMC. Teferi, Mage of Zhalfir has the Flash keyword and gives your other creature cards Flash as well. The most used card across Erayo decks is Arcane Laboratory, which limits all players to one spell each turn.

Let’s check our next Commander on the list and see if we can flesh out a strategy.

Rashmi, Eternities Crafter

While Rashmi has a lot of Rules text going on, the first clause about casting your first spell each turn matches Vial Smasher’s text verbatim. These results should be very applicable to Vial Smasher since some of the Commander’s mechanics work the exact same way.

The Signature Cards are mostly blue while the Top Cards start adding green. There is even more Delve and Flash here. Mind's Dilation stands out since the first clause is very similar to what our Commander is doing and we might get to cast a spell on another player’s turn for free.

What can we deduce so far?

Optimally, we want to cast one spell on every player’s turn. We can do this with low CMC instants, but they don’t have to be in blue. You can go back to Gatherer and spend some time digging for those in other colors to find things you like. Anything with Flash already on it can be cast on your opponent’s turns, or adding Rashmi’s Leyline of Anticipation and Vedalken Orrery will allow us to do this with all the rest of our cards.

Not all instants are low CMC, but the Delve keyword tells us that we can find cards that are cheaper to cast than they look. Gatherer can show us Delve cards outside of blue, such as Murderous Cut.

Cards that untap your lands when played will give you more mana to cast something else on your next opponent’s turn. Digging down into Erayo’s EDHREC results, by clicking on Cloud of Faeries we can see what other cards are often played in the same decks. Peregrine Drake and Palinchron appear in the Signature and Top Cards and have stronger effects of the same type.

Also in these results is Mulldrifter with the Evoke keyword. This is yet another way to make something cheaper than it looks. Paying alternative costs like Evoke doesn't change their Converted Mana Cost (CMC) on the stack, so if you Evoke Mulldrifter for 1U, Vial Smasher still does the full 5 CMC's worth of damage to an opponent. Going back to Gatherer, we can find other useful Evoke creatures in Vial Smasher’s colors such as Shriekmaw and Ingot Chewer.

Cast Spell? Deals Damage!

Now let’s try a different Gatherer search for Vial Smasher’s damage effect.

Rules Text: “When cast spell deals damage” gives us a few more Commanders to check out.

• Chandra, Fire of Kaladesh
• Hixus, Prison Warden
• Ishi-Ishi, Akki Crackshot
• Kaervek the Merciless
• Ruric Thar, the Unbowed
• Tibor and Lumia
• Vial Smasher the Fierce
• Yidris, Maelstrom Wielder

Skipping ahead a bit…

Kaervek the Merciless

This has something like Vial Smasher’s effect but it happens when your opponents cast spells instead of you. Kaervek’s EDHREC results show a lot of punishing cards that hurt players when they do literally anything. There are some damage doublers and a couple of cards that cut life totals in half.

While Kaervek’s Signature Cards might all be fine to use in a Vial Smasher deck, at this point we’re just taking a Kaervek deck and putting Vial Smasher in the driver’s seat… which might not be a bad idea in itself. Let’s try to expand what we get from these results into the other colors.

Blasphemous Act stands out as another card that gets a heavy mana discount from the printed CMC. Taking the wording “costs less to cast for each” back to a Gatherer search, we find a lot of blue Affinity cards. One possibility could be pairing Vial Smasher with Silas Renn, Seeker Adept and going heavy on Artifacts. Aside from that we also find some useful cards with the Undaunted keyword as well as standouts such as Primeval Protector and Emrakul, the Promised End.

Ishi-Ishi, Akki Crackshot